Tyre size and does it matter?

A One Tyres - Monday, September 02, 2013

Tyre sizing can be a confusing array of numbers and letters, with jargon phrases like “low profile”, “load index”, “speed ratings”. What difference does tyre size make? Well, let me break it down a bit.

How do I find out what size my tyre is?

Looking for the easiest way, call in at our depot and our staff will do the number crunching for you. But if you prefer to let your fingers do the walking, lift the phone and I, or some of the staff here can help you with what might suit your vehicle. However, if you are the more independent type and want to do it for yourself, you should look on the sidewall of your tyre, you will see a series of letters and numbers. They will be broken down into a series like 185/60 R 15 84H.


What do these numbers mean?

185 indicates the width of the tyre in millimetres

60 is the tyre’s height (ie measurement of bottom of tread to the rim) expressed as a percentage of its width.

R means the tyre has radial ply construction which is almost always found on car tyres

15 is the diameter of the rim in inches

84 is the load index and indicates the maximum load capacity for your vehicle

H indicates the speed for safe driving (H is rated for speeds up to 130mph)

What does low profile mean?

Low profile tyres are often used for high performance vehicles. Low profile simply means the size of the edge of the tyre. Narrower sidewalls can deliver better cornering, and some people think they look cool. If your vehicle doesn’t come with low profile tyres as standard, it may be possible to get low profile tyres fitted, but if you make that change you need to be careful. If you are changing your tyre size or profile, this can have an effect on your comfort, speedo, fuel consumption and can be harder on your vehicle’s suspension as well. Please phone us for more advice.

What is important?

It is important to keep the diameter of your tyre, if not exactly the same, as close to the original as possible, so that the drive shaft remains stable. If you want to make any change to your tyre size you should take advice on what is safe.

If your car is a people carrier, it is likely to have an “extra load” index. So ensuring your tyre suits the right load capacity is key to getting that right.
Matching tyre sizes is important. What you have on your front driver side, you should also have on your front passenger side. This just makes sense when you think of the tyres being part of your car’s suspension. What if my tyre is a runflat?

The size of the tyre does not change if your tyre is a runflat, although you should always seek to put the same type of tyre matching each side of your vehicle.

Still confused?

I must admit, this is a befuddling mix of millimetres, inches, percentages and other confusing indexes, but you don’t need a degree in maths to get it right. We can do it for you. Whether you have no idea of what tyre size you require, or whether you want to discuss what is possible on your car, for low profiles or other differences, we can help. Just give us a call at our Dromore depot.